With the technology development and growing demand for mobile devices, secondary batteries as an eco-friendly alternative energy source are in ever increasing demand. Recently, secondary batteries are being used as a power source for devices requiring high capacity power, for example, electric vehicles (EVs) and hybrid electric vehicles (HEVs), and their application is expanding to the use as auxiliary power through the grid.
For the use as a power source for devices requiring high capacity power, because secondary batteries must achieve high output in a short time and be used for 10 years or longer under harsh conditions of repeated charge and discharge with a large current for a short time, high energy density, outstanding safety, and long-term life characteristics are absolutely required.
Existing secondary batteries generally use lithium metal for the negative electrode, but since shorted batteries caused by dendrite formation and consequential explosion risks were reported, carbon-based compounds replace lithium metal because carbon-based compounds allow reversible intercalation and deintercalation of lithium ions while maintaining structural and electrical properties.
Carbon-based compounds have a very low discharge potential of about −3V on the basis of standard hydrogen electrode potential, and due to uniaxial orientation of a graphene layer, they exhibit extremely reversible charge/discharge behaviors and have outstanding electrode cycle life characteristics. Furthermore, an electrode potential is 0V for Li/Li+ during Li ion charging, and the potential is nearly similar to pure lithium metal, so an advantage is that higher energy is provided when a battery is made of an oxide-based positive electrode together. The negative electrode for a secondary battery is manufactured by a method which mixes a negative electrode active material 13 or carbon material with, if necessary, conductive material and binder to prepare a negative electrode active material slurry, applies a layer of the slurry to an electrode current collector 11 such as a copper foil and dries the slurry. In this instance, when applying the slurry, a press process is performed to compress active material powder onto the current collector and achieve uniform electrode thickness.
However, in the conventional electrode press process, stronger pressure is applied to the surface of the negative electrode active material than the inside, and the pore volume fraction on the surface reduces. This phenomenon is worse as the electrode is thicker, and permeation of an electrolyte solution into the electrode is poor and channels for ion movement are insufficient to achieve smooth ion movement, resulting in degradation in battery performance and life characteristics.
On the other hand, when an electrode is composed of a layer of active material as conventionally, the layer of negative electrode active material is soft and vulnerable to stress, and due to these properties, the pressure cannot be transmitted to the inside of the electrode and only the negative electrode active material disposed on the electrode surface is strongly pressed during a press process. For example, when an electrode is only formed from a layer of active material having a low press density and a large average particle size, the layer of negative electrode active material is vulnerable to stress, and due to these properties, only the negative electrode active material disposed on the electrode surface is strongly pressed during a press process. As a result, pore spaces in the negative electrode active material near the electrode surface reduce and migration of ions toward the electrode reduces. This phenomenon will be worse if the negative electrode is thicker or denser.